Frazil ice is formed in turbulent, supercooled water. It is normally well distributed throughout the depth of the flow, ranging in size from 50 microns to several centimeters. Frazil ice can adhere to underwater objects. In the winter season, it commonly causes blockage of water intakes by adhering to the water intake trash rack. Prior to this invention, there is no device for the detection of underwater frazil ice. Previous methods used for detection included inference by measuring the rate of change of water temperature near the freezing point, monitoring pressure head loss or discharge reduction due to freeze up of an inlet, or monitoring meteorological conditions. None of the aforementioned methods provides an unambiguous means of detecting frazil ice.
Related prior art discloses ice detectors for use on an aircraft, as illustrated by Hahn, U.S. Pat. No. 2,541,512; Timbie, U.S Pat. No. 2,739,302; and Vykhodtsev, U.S. Pat. No. 4,210,021. Other related art discloses devices which sense the clogging of filters due to pressure change. Illustrative references are Isely, U.S. Pat. No. 4,263,805; Miller, U.S. Pat. No. 3,686,926; and Anderson, U.S. Pat. No. 3,077,988.